This invention relates to an apparatus and method for sensing and controlling the concentration of toner in a developer mix used in a xerographic or the like copier to develop electrostatic images.
It is well known in the art that the proper concentration of toner in a developer mix comprising both toner and carrier particles used to develop latent electrostatic images produced on a photo-conductive or the like medium in a xerographic-type copier is important to the quality of the copies provided. Many different types of devices are known in the art for monitoring and controlling the toner concentration of a developer mix in a copier. One well known device uses Nesa glass over which a sample of developer mix is passed. The Nesa glass is provided with a pattern which is charged and developed by the developer mix. Light passing through the Nesa glass subsequent to development indicates the density of the toner and therefore the concentration thereof in the mix. The latter is used to control the replenishment of toner to the mix in the developer apparatus of the copier.
Other devices are also known in the art for determining the toner concentration of a developer mix. These devices employ circuitry to measure the resistance or inductance of the mix and thereby determine its toner concentration.
While the above-described devices work satisfactorily for the most part to provide an indication of the concentration of toner in a developer mix comprising both toner and carrier particles, they have certain drawbacks which make them less desirable. In the case of the first-described device, sample development with the mix is required to make the determination of toner concentration. Thus, after the glass is developed, it must be cleared of mix for the next sampling. Also, light sources and coated Nesa glass material is needed which can be expensive. In the case of the resistance or inductance measuring devices, relatively complex circuitry is required to perform the operation.